A low-complexity implementation of a transmitter design for transceivers in mobile radio systems is provided by transmitters which have a modulator operating on the basis of the known principle of two-point modulation. In this case, a PLL (Phase Locked Loop) circuit is used as a frequency synthesizer and is used for phase or frequency modulation of a radio-frequency signal.
The modulation signals are usually impressed at two points in the PLL circuit. First, a programmable frequency divider in the PLL circuit is actuated by a digital modulation signal. The programmable frequency divider is fitted in the PLL circuit's feedback path and represents a point in the PLL circuit at which a low-pass response is obtained for impressing a modulation. In this case, the digital modulation signal can have a larger bandwidth than the low-pass filter formed by the PLL circuit. Secondly, an analog modulation signal is injected into a summation point which is situated in the PLL circuit's forward path and is preferably connected upstream of the voltage controlled oscillator. The analog modulation supplied at the summation point has a high-pass filtering effect on the output of the PLL circuit on account of the closed control loop, which means that the corresponding modulation signal is in turn corrupted by the response. The digital and analog modulation signals are superimposed on one another at the output of the PLL circuit, and in this way a frequency-independent response for the PLL circuit is obtained. The simultaneous impression of a digital and an analog modulation signal into a PLL circuit is called two-point modulation.
Such a two-point modulator and a method for phase or frequency modulation with a PLL circuit is described in German laid-open specification DE 199 29 167 A1. A digital modulation signal is supplied to the control connection of a frequency divider in the PLL circuit's feedback path, and this determines the number by whose reciprocal the instantaneous frequency of the input signal for the frequency divider is multiplied. In addition, the digital modulation signal is converted by means of a digital/analog converter into an analog modulation signal which is injected into the PLL circuit at a summation point which represents a high-pass point. This laid-open specification can be regarded as the closest prior art to the present invention.
In the case of the described type of transmitter design, the control loop remains closed. To achieve low noise in the PLL circuit, the bandwidth of the PLL circuit is designed to be much smaller than would be necessary for transmitting the modulated data. To compensate for the small bandwidth, the analog modulation signal is injected into the PLL circuit in addition to the digital modulation signal.
Both the digital modulation signal and the analog modulation signal are prefiltered in order to limit the PLL circuit's output spectrum. The digital modulation signal is usually filtered by means of oversampling, i.e. one bit is represented by a plurality of samples. In addition, for two-point modulation to work correctly, it is necessary for a high degree of correspondence between the amplitudes of the two modulation signals to be ensured in addition to phase equality over time. In the case of known two-point modulators, filters having the same pulse shaping are used for the digital and analog prefiltering for this reason. A drawback of this is the correspondingly high implementation complexity for such digital and analog filters.